Python pip: Installing specific tags or versions from Github

Installing packages from pip is not just convenient, it’s also a requirement when working from a virtualenv. Unfortunately, there comes a time when installing a specific version of a package requires a little more finess–and often some mucking about on Github. While there are solutions to this dilemma, the cleanest and most concise one I’ve yet discovered is to modify a particular Github trick that’s documented both on Github and in a number of Python package repositories that are not presently available on PyPI:

pip install https://github.com/<username>/<package>/tarball/<tag, branch, or commit>

For example:

pip install https://github.com/surfly/gevent/tarball/1.0rc3

That’s it!

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jQuery MSIE7 Attribute Traversal/Clone Crash Bug

I’ve been spending some time writing a client-side JavaScript library (more on this in a later post) that does a fair amount of DOM manipulation and has had its share of cross-browser, erm, idiosyncrasies. The foundation of the library depends rather heavily on jQuery for traversal, DOM insertion and removal, and the likes. It’s also fully unit-tested and therefore it’s relatively trivial to wire-in another browser, run up the unit tests, and see what works (or doesn’t). Unsurprisingly, every browser except most flavors of MSIE have worked without issues, but many of the MSIE issues I encountered have relied on minor workarounds for missing features or misbehavior that’s more noisome than frustrating.

That is to say until I discovered it’s possible to crash MSIE7 using a mix of traversal, detaching, and cloning. Needless to say, the unit tests barely finished, and I was left surprised. I’ve since fixed the problem, but it’s nevertheless been a curio in need of a simplified test case.

First, I want to point out that I suspect this may be tangentially related to this bug, although I’m reluctant to call it a jQuery “bug” considering that it works fine in every other version of MSIE (including 6 and ever other browser I’ve tested). Furthermore, since the release of jQuery 2.x drops support for MSIE 6, 7, and 8, the importance of addressing this “bug” is largely moot. Most applications, particularly single-page client-side applications that perform a great deal of work directly on the DOM, target newer browsers anyway. Only those with a large or predominantly non-technical audience ought to worry, and many of those sites likely don’t make use of much JavaScript outside advertising and analytics.

The bug manifests itself whenever the user manipulates the .data() method on an element and subsequently detaches that element and clones it. Actually, that’s a lie: Detaching and .data() manipulation can occur in any sequence. The important bit to take from this is that any manipulation of .data() on an element that has been or will be detached and further cloned will duplicate this behavior. Here’s a short example.

Given the HTML:

<div class="crash-me">Loop over this node's data, detach it, and clone it
to crash MSIE7.</div>

And the JavaScript (using jQuery):

var $element = $(".crash-me");
 
// Looping over data elements. The crash will occur even if they're empty.
$.each($($element[0]).data(), function(key, value){
  $("#values").append(key+": "+value+"<br>");
});
 
// Now to detach.
$element.detach();
 
// Crash.
for (var i = 0; i < 5; i++) {
  // You won't see the clones in MSIE7 'cause it's dead.
  // Technically, you only need the $element.clone() call.
  $("#nodes").append($element.clone());
}

You can also replace the $.each with a for-loop to the same effect:

var attributes = $element[0].attributes;
$($element[0]).data("test", 1);
for (var i = 0; i < attributes.length; i++) {
  // You only need to access nodeName or nodeValue to do this horrible deed.
  if (attributes[i].nodeName) {
    $("#values").append(attributes[i].nodeName+": "+attributes[i].nodeValue+"<br>");
  }
}

The difference, however, is that the $.each loop will trigger the crash whether or not the inner function does anything (since the nodes’ attributes have already been accessed); the for-loop requires that you do something with attributes, such as fetching the nodeName or nodeValue. This is essentially what $.each does anyway, so the two methods accomplish roughly the same thing.

The workaround for this is to simply avoid touching .data() on any node that is (or will be) detached from the DOM and operate only on the clones. In my case, I have a traverse() method that traverses over all DOM elements, looking for data-* attributes, and then examines their children for more data-* elements. To avoid this bug, I have a flag for the attribute handlers that indicates whether or not it detaches its children; if it does, traverse() simply ignores it and carries on with its business. It’s up to the handler to call traverse() on the clones of its detached children, never operating directly on the original children themselves. Since each handler that relies on traversal uses the same traverse() method for DOM manipulation, it’s easy to manage the fix in a single location.

Of course, if you have little choice but to examine the data attributes of a detached node before cloning them, you’re be out of luck. Fortunately, MSIE7 usage is declining, and if you’re outside the financial sector or government, you should be safe! My condolences otherwise.

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No Decode Delegates… and Conflicts

It’s not often that we run into unusual conflicts rooted in hard to find places, but those few times that they are encountered–no matter how rare–can be infuriating. I was recently bitten by this one, demonstrated by a particularly bizarre error when attempting to load an image (any type) into the php-imagick ImageMagick extension:

$ php -a 
Interactive shell
 
php > $c = new Imagick('kmix.png');
PHP Warning:  Uncaught exception 'ImagickException' with message 'no decode delegate for this image format `kmix.png' @ error/constitute.c/ReadImage/552' in php shell code:1
Stack trace:
#0 php shell code(1): Imagick->__construct('kmix.png')
#1 {main}
  thrown in php shell code on line 1

Yet, paradoxically, convert -list delegate, convert -list configure, and convert -list format all hinted that PNG and JPEG support were both installed. Worse, searching for “php imagick no decode delegate” resulted in few useful hits, and of those hits most were indicative of configuration problems, custom builds missing the appropriate libraries, and other dilemmas punctuated by the usual PEBKAC retort (though such accusations are sometimes accurate).

Nothing worked, of course. Swapping out to older versions of ImageMagick failed, and I had no intention of trying an earlier version of PHP or php-imagick. At this point, I had no expectations to find the solution online, and the only thing I surmised might work would be to attempt duplicating the problem on another machine. So, I set about installing ImageMagick (v6.8.5-10) and php-imagick (v3.0.1 plus PHP 5.4 patches and v3.1.0RC2) with the current-latest version of PHP available in the Arch repositories (v5.4.16) and quickly discovered that it worked quite well. This was a positive sign: Nothing about the latest builds indicated there was a problem, so it was plausible that the problem lurked elsewhere.

And it did.

I soon noted that the virtual machine I booted was lacking php-gmagick and GraphicsMagick both, though ImageMagick and GraphicsMagick worked quite well on my machine together. Thus, I speculated that perhaps php-gmagick was conflicting somehow with php-imagick. I commented out the gmagick extension, re-ran some unit tests, and presto! php-imagick was now working perfectly fine. Uncommenting the gmagick extension resulted in a return of the problem, and so I was (and am) convinced that the problem is a conflict between both extensions. Possibly, this is due to the definition of symbols or other such nonsense in gmagick that imagick requires (and is very upset when it discovers they already exist).

Is it possible to get them to play nicely together? Maybe.

On my machine, PHP extensions and their options are loaded from /etc/php/conf.d. These files are, apparently, loaded alphabetically in order, and since “G” comes before “I,” you might imagine (correctly) then that gmagick gets loaded before imagick which is where the magic stops (my apologies; that pun was awful). By renaming them to something creative, such as “10-imagick.ini” and “20-gmagick.ini”, it is then possible to force imagick to load before gmagick, and they’ll both work cooperatively near as I can tell. Perhaps gmagick defines–but doesn’t use–some interpreter symbols that imagick actually needs, but if they’re already defined gmagick doesn’t care? I don’t know. I can only speculate, because I know next to nothing about PHP internals. (Perhaps now might be a good time to start.)

Although renaming or otherwise moving the extension configs into an order that would suggest imagick.so gets loaded first seems like an adequate solution, the problem is that these two extensions most assuredly do conflict. Between PHP versions, you can’t be guaranteed that the loader mechanism for extensions will remain static and thus the possibility exists that renaming the *.ini files is only an ephemeral solution. Moreover, it’s also possibly an accident that it works on my machine; perhaps the inode order is such that imagick.so now gets loaded before gmagick.so simply due to luck. Or maybe I’ve now reduced it to a race condition? The point is that you shouldn’t rely on this in production. Instead, you should select between GraphicsMagick or ImageMagick, picking the one that best fits your use case. If you’re using a wrapper library to select the best backend available, force it to load a specific one in production mode. Obviously such concerns aren’t much of a bother on a development box, but I’d nevertheless recommend a healthy dose of caution.

If you’ve encountered this issue yourself but never found a reasonable solution, let me know. If this article helps in any measurable way, I’d be happy to hear from you. It’s been a frustrating few hours for me, and I hope that this will at least alleviate you of wasting further time on something that is poorly documented but easy to fix.

2 comments.
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